---
name: library-logger-warn
group: api
category: facade
update-time: 20260613
description: Emit a warn-level record through a LibraryLogger facade by delegating to the wrapped logger's warn shortcut.
key-word:
    - library
    - facade
    - warn
    - public
---

## Library-logger-warn

Emit a warn-level record through the library-facing sync logger. This is the convenience wrapper for `log(Level::Warn, ...)` on `LibraryLogger[S]`.

### Interface

```moonbit
pub fn[S : Sink] LibraryLogger::warn(
  self : LibraryLogger[S],
  message : String,
  fields~ : Array[Field] = [],
) -> Unit {
```

#### input

- `self : LibraryLogger[S]` - Library-facing logger that should emit the warning record.
- `message : String` - Warning message text.
- `fields : Array[Field]` - Optional structured fields attached to the record.

#### output

- `Unit` - No return value. The record is handled according to the current threshold and wrapped sink pipeline.

### Explanation

Detailed rules explaining key parameters and behaviors

- This helper delegates to `warn(...)` on the wrapped logger, which in turn uses `log(Level::Warn, ...)`.
- Warning records are useful for abnormal but non-fatal conditions.
- Per-call target override is not exposed here; use `log(...)` when that is required.
- All sink-side wrappers still participate normally in the write path.
- Broader composition helpers remain on the underlying `Logger[S]` and require `to_logger()` first.

### How to Use

Here are some specific examples provided.

#### When Signal A Recoverable Library Problem

When an operation degraded but still continued:
```moonbit
logger.warn("cache miss ratio increased")
```

In this example, the event is elevated above normal information without being treated as a hard failure.

#### When Attach Structured Warning Context

When a warning should include machine-readable detail:
```moonbit
logger.warn("retry scheduled", fields=[field("attempt", "3")])
```

In this example, the warning remains easy to filter and inspect later.

And any shared context already carried by the facade still participates through the wrapped logger pipeline.

### Error Case

e.g.:
- If the logger minimum level is above `Warn`, the call returns without writing a record.

- If a target override is needed at this call site, use `log(...)` instead of this shortcut.

- If callers need broader composition helpers after logging, they must unwrap first with `to_logger()`.

### Notes

1. Use this helper for degraded or suspicious states that do not stop execution.

2. Warning logs are often a practical signal threshold for alerting or separate routing.

3. Use `log(...)` instead when the call site needs a per-call target override or a dynamic level.